Study On Flow Characteristics And Heavy Metal Leaching Toxicity Of Municipal Solid Waste Ash Slag

The composition of municipal solid waste is complex,and the traditional treatment methods have low energy efficiency and are prone to secondary pollution.Fixed bed slag gasification has the advantages of raw material adaptability and ash treatment,and it is the first choice for resource utilization and harmless treatment.The operating conditions of the fixed bed slag gasifier are closely related to ash fluidity and also affect the leaching toxicity of heavy metals.In this thesis,ash fusion and viscosity-temperature characteristics of four kinds of municipal solid waste(MSW)ash slag under gasification conditions were studied.The physicochemical characteristics of the gasified ash slag in the pilot plant of fixed-bed slag gasifier were analyzed.The mechanisms of ash fluidity with the addition of SiO2 were investigated.And the effects of chemical components,melting temperature and melting process on the leaching toxicity of heavy metals were investigated.The main research findings are as follows:(1)The chemical composition of the four kinds of ash is mainly composed of SiO2,CaO,Al2O3,and Fe2O3,of which the silicon and calcium contents are high.The mineral composition is mainly quartz and calcite.The ash flow temperature(FT)is less than 1350℃.The melting behavior is consistent with the "melting-dissolution" mechanism.The low melting point is related to the low-temperature eutectic of calcium-based minerals such as wollastonite,calcium-aluminum feldspar and calcium-magnesium feldspar in the ash slag.The viscosity-temperature curves of the slag show that Shanghai garbage(LG),Ningbo garbage(NB)and Yangzhou garbage(YZ)are glass slag,while the pilot gasification feedstock(YY)is crystalline slag.The size and number of crystals such as calcium feldspar and spinel affect the growth rate of slag viscosity during the cooling process.The addition of SiO2 changes the fluidity of YY.As the ratio of SiO2 increases,the FT of YY decreases and then increases,and the slag type changes from crystalline slag to plastic slag and then to glass slag.(2)The particle size of the pilot gasification ash slag(ZS)was mainly concentrated in the range of 0.43～7.00 mm.The overall vitrification degree of slag is high.The chemical composition of slag with different particle sizes is similar,and the main components are SiO2 and CaO.When the particle size<0.25 mm or>7 mm,there is still a small amount of combustible material,quartz and calcium-magnesium yellow feldspar in the ash slag.In different particle sizes of slag,the content of Ni,Cr,Cu and Zn is higher,and the content of Cd and Pb is lower.The leaching concentrations of Cd,Pb,Zn,Cu and Cr are lower than the limits specified in the pollution control standard for municipal solid waste landfills.However,the leaching concentration of Ni only in 5.00～7.00 mm particles is lower than the standard limit.(3)The melting temperature has a significant effect on the leaching concentration of heavy metals than the addition of SiO2.With the increase of temperature,the overall trend of Cr,Zn and Cu leaching concentrations decrease.At the same melting temperature,increasing the addition of SiO2 facilitates the formation of stable compounds of Pb,Zn and Cu in the slag.When the SiO2 addition ratio is 0～30%,the leaching concentrations of Cr,Zn and Cu in slag at 1300℃,1400℃ and 1500℃ meet standard,and the leaching concentrations of Ni in slag at 1400℃ and 1500℃ meet standard.The leaching concentration of Pb does not only depend on melting temperature,but also needs to adjust melting process in order to meet standard.