Migration And Distribution Of Heavy Metals Cd And Zn In Waste Incineration Process And Thermodynamic Simulation

In this paper,the actual municipal solid waste of Shanghai Laogang Waste Incineration Plant was used as the experimental raw material,and the SK2-4-12 tube furnace was used to simulate the domestic waste incineration process to study the migration characteristics of heavy metals Cd and Zn during waste incineration.The additives used were chloride(PVC,NaCl and KCl),sulfur and sulfide(S,Na2S and Na2SO4),oxides(CaO,MgO,SiO2 and Al2O3)that were investigated at different temperatures(600℃,700 ℃,800 ℃ and 900 ℃)on the migration and distribution of Cd and Zn under different loading conditions.The morphology distribution of Cd and Zn was calculated by HSC Chemistry 6.0 thermodynamic simulation software,and the migration characteristics were further verified and analyzed.Chloride can promote the volatilization of Cd and Zn.The volatilization effect will be better with the increasing temperature and the amount of addition.For Cd and Zn,the effects of volatilization are all PVC>NaCl>KCl at the four temperatures.The results of thermodynamic equilibrium calculation show that the gas phase forms of Cd are CdCl2(g),Cd(g),CdCl(g)and Zn are ZnCl2(g),Zn2Cl4(g),ZnCl(g),Zn(g).Therefore,the gas phase and chloride of heavy metals will be much more with the increasing temperature and Cl concentration.It can be confirmed that the temperature and Cl concentration determine the effect of the chloride.The effect of sulfur and sulfide on Cd is inhibited at 600 ℃ and 700 ℃,and the inhibitory effect is Na2S>S>Na2SO4.At 800℃,Na2SO4 turns to promote Cd volatilization,however,S and Na2S inhibit Cd volatilization,and inhibitory effect is S>Na2S.At 900 ℃,S still inhibits Cd volatilization,and Na2S promotes Cd volatilization as well as Na2SO4.For Zn,sulfur and sulfide can inhibit Zn volatilization,the inhibition effect will be better with the increasing temperature and the amount of addition.Under the same temperature condition,the inhibition effect is Na2S>S>Na2SO4.The results of thermodynamic equilibrium calculation show that CdSO4 and(CdO)2*CdSO4 will be decomposed into solid CdO at 800 ℃.The sulfate of Cd is unstable and easy to decompose.The sodium sulfate fluxing is easy to dominate and promote the evaporation of Cd.It exists in the form of ZnO and ZnSO4 at high temperature,and it is confirmed that the sulfate of Zn is stable,and the effect of sodium sulfate is not obvious.Oxides inhibit the volatilization of Cd and Zn.The lower the temperature,the larger the amount added,and the better the suppression effect.For Cd,the inhibition effect is SiO2>Al2O3>MgO>CaO at 600 ℃.The inhibition effect of SiO2 and Al2O3 is similar at 700 ℃.Al2O3>SiO2>MgO>CaO at 800 ℃ and 900 ℃.For Zn,the inhibition effect is SiO2>MgO>Al2O3>CaO at 600 ℃.the effect of MgO and SiO2 is close at 700 ℃.MgO>Al2O3>SiO2>CaO at 800 ℃ and 900 ℃.The results of thermodynamic equilibrium calculation show that SiO2 and Al2O3 react with Cd and Zn respectively to form non-volatile CdSiO3,CdAl2O4 and ZnAl2O4,ZnSiO3.CaO and MgO can combine free chlorine and HCl in the system and reduce the concentration of Cl,inhibiting Cd and Zn volatilization.In addition,at 800 ℃,the mixed addition of Al2O3 and SiO2(5.0%of the mass of the waste)inhibites the volatilization of Cd and Zn more strongly than the addition alone.When the ratio of silicon to aluminum is 1:2,the distribution of Cd in the bottom slag reaches the maximum,which is 10.83%.When the ratio of silicon to aluminum is 1:1,the distribution of Zn in the bottom slag reaches the maximum,which is 79.48%.