Transfer Characteristics Study Of Heavy Metal In The Thermal Treatment Of Sewage Sludge

With the wastewater discharge amount increasing and the development of wastewater treatment, the quantity of sludge is growing quickly in China. Sludge thermal treatment has broad application prospects due to the advantages of significantly reducing the volume of sludge, killing organic micro-organisms and saving energy. The emitting of heavy metals which harmful to humans and the environment, seriously restrict the development of sludge heat-treatment. In this paper, thermodynamic equilibrium analysis is used to predict heavy metal speciation transformation and distribution during thermal treatment. The migration characteristic of heavy metals and the effects of chlorine, sulfur and phosphorus on the migration of heavy metals during sludge thermal treatment were researched in detail based on the combination theoretical and experimental method.This paper simulated a sewage sludge system based on an actual measured data. The thermodynamic equilibrium calculations by HSC-Chemistry 5.0 software were performed to reveal the speciation transformation and distribution of Cu, Cd, Pb, Zn, Ni and Cr during sludge thermal treatment. First this paper studied the speciation transformation and distribution of heavy metals during sludge thermal treatment, and then studied the effects of the different contents of chlorine, sulfur and phosphorus on the speciation transformation and distribution of heavy metals during thermal treatment. The results showed that heavy metals were based on solid sulphate states at a lower temperature. The sulfates of Cu, Ni, Cd and Cr were decomposed into solid oxide, and the sulfates of Pb and Zn were converted into solid phosphate. At a high temperature, the heavy metals existed as the forms of gaseous metal substances or gaseous metal oxides. Chlorine was easy to form chloride with heavy metals and accelerated heavy metals transfer from solid phase to gas phase while sulfur presented opposite trend because S could form the solid sulfates. Phosphorus had great influence on the speciation transformation and distribution of Pb and Zn, and inhibited Pb, Zn transfer from solid phase to gas phase with forming phosphate.On the base of chemical thermal equilibrium calculation, the effects of chlorine, sulfur and phosphorus on the transformation of heavy metals were studied on the tube furnace. The results of experiment showed that Chlorine accelerated Cd, Pb, Zn and Cu transfer from solid phase to gas phase and had less influence on Ni and Cr. Sulfur had less influence on the transfer characteristics of heavy metals, and inhibited Zn, Cd, Ni transfer from solid phase to gas phase while presented opposite trend on Pb in a certain extent. Phosphorus had different influences on the transfer characteristics of heavy metals. With the increasing contents of phosphorus, Cr and Ni volatilization rates increased while Cd, Pb and Zn volatilization rates decreased gradually.