Study On Treatment And Utilization Of MSWI Fly Ash By Molten Salt Method

With the advantages of volume reduction,weight reduction,high efficiency and the second use of thermal energy,Municipal solid waste incineration(MSWI)technology has been widely applied in China.MSWI fly ash is produced in the process of municipal solid waste incineration and contains a lot of heavy metals,dioxins and other toxic pollutants.So MSWI fly ash should be treated harmlessly.In recent years,the treatment technology for MSWI fly ash harmlessness and resourcefulness is one of the research hotspots in the waste disposal.The chemical and physical properties of the MSWI fly ash samples,such as chemical composition,the surface microstructure and leaching characteristics of heavy metals were sueeessively analyzed by X ray diffraction,X ray fluorescence spectroscopy,scanning electron microscope and inductively coupled plasma mass spectrometry.In this paper,MSWI fly ash was used to produce building materials by steam pressure curing technology.The study found that the strength of the solidified body could meet the requirements of building materials when the addition of MSWI fly ash was less than 45.0%,and the addition of fly ash was higher than 50.0%.And the heavy metal leaching toxicity of the solidified bodies were far less than limits stipulated in the standards.The MSWI fly ash was treated by molten salt method as heavy metals were easy to chlorinated and volatilized at high temperature.The influences of different treatment temperature,molten salt composition,different proportion of molten salt and different atmospheres on the volatilization performance of heavy metals are studied.The basic properties and resource utilization of molten salt treated MSWI fly ash were analyzed.With the increase of NaCl content and temperature,the heavy metal leaching toxicity of molten salt treated MSWI fly ash was decreasing.Adding a certain amount of calcium chloride in reducing atmosphere could effectively promote the volatilization of heavy metals in MSWI fly ash.The optimum reaction conditions obtained by the experiment were showed as follows: the treatment temperature is 1000 °C,the molten salt is composed of sodium chloride and calcium chloride with the mass ratio 1:1,the mass ratio of molten salt and MSWI fly ash is 10:1,1 g toner was added,and treatment temperature is 2 h at the reduction atmosphere.Under this condition,the heavy metals Cr,Cu,Zn,Cd and Pb leaching toxicity of molten salt treated MSWI fly ash were 62.80 μg/L,69.40 μg/L,493.90 μg/L,2.10 μg/L and 49.90 μg/L,which were far less than limits stipulated in the standards.Molten salt treated MSWI fly ash mainly consisted of tricalcium silicate,dicalcium silicate,alinite,calcium sulfate and other minerals,which was similar to the composition of cement,so it could be mixed into cement in different proportions.It was found that with the increase of molten salt treated MSWI fly ash content,water consumption of normal consistency consistency increased,the setting time increased initially and then decreased,also the bending strength and compressive strength increased initially and then decreased.When the content of molten salt treated MSWI fly ash is 10%,the compressive strength of the 3 days,7 days and 28 days could reach the maximum,which were 22.5 MPa,32.9 MPa and 43.6 MPa respectively.When the content of molten salt treated MSWI fly ash is 30%,the 28 days compression strength ratio with non-additive cement is 87.7%,which was more than 65%.It showed that the molten salt treated MSWI fly ash had good gelling property.