Research On The Leaching Characteristics And Leaching Process Control Of Heavy Metal In Waste Incineration Fly Ash

With the rapid development of waste incineration treatment and the more intensive shortage of land resources,the disposal of fly ash from waste incineration has gradually become a bottleneck in the environment friendly development of cities.There is a large amount of heavy metals such as Pb,Zn,Cd,Cr,the compounds containing heavy metals and soluble inorganic salts in fly ash,which makes it become a sort of hazardous substance with the high environmental risk.The heavy metals in the fly ash are mainly from municipal solid wastes include newspapers,plastics,pigments,nickel-cadmium batteries and thermometers.Though the solid waste incineration with high temperature circumstance,the heavy metal can be concentrated in the fly ash,which lead the concentration of heavy metal in it to become several times higher than original waste.The components of heavy metal in fly ash are determined by many factors such as flue gas of waste incineration,the components of municipal solid waste,the temperature of incineration,incineration process,the physical and chemical attribution of heavy metal and so on.Therefore,this process is extremely complex.All these make fly ash have different properties in biotoxicity,bioaccumulation,migration and leaching toxicity.In modern society,the techniques of fly ash treatment include cement/lime solidification,glass/melt solidification,chelating agent solidification,chemical treatment,sintering treatment,biological leaching and resource.Every coin has two sides.These techniques have variable advantages and disadvantages such as high cost,complex treatment and low efficiency,which limit the application.This research focus on the process of leaching heavy metals in heavy and simulates to dissolve the acidic gas such as HC1,SO2 and so on from waste incineration in water.In this way,we can make mixed acidity to leach fly ash,which is able to move heavy metals in solution.This is the core part of this treatment.At the same time,we also do research in the process of leaching heavy metals.Specially,we make use of water washing treatment,acidity washing treatment and alkali washing treatment to treat fly ash and research the efficiency of these treatment.In order to remove the heavy metals in fly ash and to decrease the environment risk of it,we dispose fly ash by different combination of these treatments.The results of experiment indicate that the different kinds of fly ash from different areas have different moisture content,burning loss rate and soluble alkali content.But all of these have effective acid-base buffer capability.The particle sizes of most fly ash particles distribute within the range of 0?200 pm which means particle sizes of fly ash is small.The main elements in fly ash are O,Cl and Ca,especially,the high Cl content dues to the fact that municipal waste has a lot of Cl from food waste containing salt and plastic waste.The main components of fly ash are CaCO3,CaSO4,SiO2 and chlorine compounds,most of which are alkali metal chloride.Fly ash contains carbonate chemical group,sulfate chemical groups and chemical bond such as Si-Si,O-Si-O and so on.There is few organic chemical functional groups in fly ash.The pores on the fly ash surface are almost slit holes formed by the accumulation of flaking particles.This structure is very complex and most pore sizes on its surface distribute between 2 nm and 50 nm,which belongs to the range of mesoporous.A large number of acicular crystals,quasi-spherical particles and many irregular aggregates-mainly metal salt crystals and glassy substances-can be observed on the surface of fly ash by electron microscope.Water washing treatment can effectively remove soluble alkaline substances,chlorine containing compounds and some heavy metal compounds in fly ash.It can also improve the leaching efficiency during the acidity washing treatment.With the increasing the ratio of water and fly ash,the removal efficiency of soluble alkaline substances also increases.When the ratio of water and fly ash arrives 10:1(L/kg),the removal efficiency can reach 98.88%which is a pretty high removal efficiency.During the acidity washing treatment,the treatment time and the ratio of acidity and fly ash can effectively affect the removal efficiency of heavy metal in fly ash.With the increasing of acidity treatment time,the removal efficiency of heavy metal in fly ash gradually increases and when the treatment time reaches 120 min,the curve of removal efficiency of heavy metals in fly tend to become stable.In essence,the variation of acidity washing ratio can affects the heavy metal leaching efficiency of fly ash by changing the pH value of the solution and we can yield a pretty good removal efficiency with the ratio 40:1(L/kg).The variation of temperature and oscillation rate in a certain range during acidity washing treatment has no obvious effect on the removal efficiency of heavy metals in fly ash.Pb and Zn in fly ash can be effectively removed in alkaline environment.The treatment of fly ash by alkali washing+water washing+acid washing process has the highest removal efficiency of heavy metals.But the water washing+water washing+acidity washing process can maintain a high removal efficiency of heavy metals and reduce the cost,which is an optimal scheme.Fly ash treated by twice of water washing can not only effectively remove metal chloride and heavy metals such as Zn and Pb from fly ash,but also can remove most soluble alkaline substances in fly ash.Most of the calcium,magnesium,aluminum,sodium and potassium chloride salts and some metal salt hydrates are soluble in water and can be effectively removed by water washing treatment.As a result of it,the surface attachment of fly ash is reduced after water washing treatment and the particle distribution on fly ash surface is more scattered.We can effectively reduce the cost of this treatment by making use of the reused water in solid waste incineration factory as this treatment water.The micro pump can continuously extract acid gas into the reused water and make it become acidic solution which can be used during acidity washing treatment.We can effectively low the environment risk of fly ash by this way though the removal efficiency of heavy metals in fly ash decreases a little at the same time.