城市污泥焚烧过程中重金属形态与分布的热力学平衡分析

Thermodynamic equilibrium analysis of heavy metals speciation transformation and distribution during sewage sludge incineration

采用化学热力学平衡分析方法,应用污泥实测数据预测4种不同来源的污泥焚烧过程中重金属Pb、Ni、Mn、Cr、Zn和Cu的迁移和转化规律。结果表明:重金属Ni、Mn和Zn在低温条件下受矿物质Al2O3、CaO、SiO2和Fe2O3的影响较大,易形成稳定的固体而存在于炉渣中;Cr易形成氧化物而基本不受矿物质的影响,Pb的形态转化受多种因素的制约且易于挥发;随着焚烧温度的升高,重金属Pb、Cu和Zn从对应的金属固态盐类或氧化物逐渐转化为气态的金属氯化物、氧化物,再到气态单质进入灰飞或大气中;Mn、Ni和Cr易形成不易挥发的稳定固体而存在炉渣中;焚烧体系中S能与Ni、Cu和Pb结合形成硫酸盐,而对Mn、Cr和Zn的形态转化影响较小;焚烧体系中Cl对Pb的挥发影响最大,其次为Cu和Zn,影响较小或者不受影响的金属是Ni、Cr和Mn。

The chemical thermodynamic equilibrium calculations were performed to reveal the migration and transformation of the heavy metals,such as Pb,Ni,Mn,Cr,Zn and Cu,during incineration of four sewage sludges.The actual sewage sludge compositions and combustion conditions were utilized in all simulations.The results show that the heavy metals Ni,Mn and Zn are affected greatly by the minerals,such as Al2O3,CaO,SiO2,Fe2O3,and form some stable solids easily which exist in the residual slag at low temperature.Under the same condition,Cr is easy to form its oxides and does not affected by the minerals,but Pb is transformed to other volatile forms easily and affected by many other factors.With increasing temperature,the heavy metals Pb,Cu and Zn transfer from their solid salts,oxides to gas metal chlorides,oxides,and finally to gas elements,which enter into the fly ash or the atmosphere.The elements Mn,Ni and Cr are easy to form some stable,high melting point and non-volatile oxides existing in their residual slags.During the incineration,the element S reacts with Ni,Cu and Pb,and forms sulfates,while the transformation of Mn,Cr and Zn affected by sulfur is less.When there is Cl in the incineration system only,the effect of Cl on the volatilization of Pb is the greatest,followed by the volatilizations of Cu and Zn,Ni,Cr and Mn has less effect or no effect.