Thermodynamic Equilibrium Calculations on Cd Transformation during Sewage Sludge Incineration

Thermodynamic equilibrium calculations were performed to reveal the distribution of cadmium during the sewage sludge incineration process. During sludge incineration in the presence of major minerals, such as SiO₂, Al₂O₂ and CaO, the strongest effect was exerted by SiO₂ on the Cd transformation compared with the effect of others. The stable solid product of CdSiO₃ was formed easily with the reaction between Cd and SiO₂, which can restrain the emissions of gaseous Cd pollutants. CdCl₂ was formed more easily in the presence of chloride during incineration, thus, the volatilization of Cd was advanced by increasing chlorine content. At low temperatures, the volatilization of Cd was restrained due to the formation of the refractory solid metal sulfate. At high temperatures, the speciation of Cd was not affected by the presence of sulfur, but sulfur could affect the formation temperature of gaseous metals.