Stabilization Of Heavy Metals In Fly Ashes From Typical Municipal Solid Waste Incineration Via Mechanical Milling

With the rapid growth of municipal solid waste?MSW?in our country,it is worth to consider how to deal with the lots of MSW harmlessly and effectively because of the impact on citizens'daily life and sustainable development.Incineration has been used widely to treat MSW due to its significant advantages of capacity reduction and resource utilization.But we can not ignore the heavy metal pollution in fly ashes from municipal solid waste incineration?MSWI?.Mechanical milling is a method of treating solid particles,which is easy to be operated efficiently.So it has been gradually applied to treat hazardous waste in recent years.In this study mechanical milling was used to stabilize the heavy metals in fly ashes and control the leaching of heavy metals in fly ashes.Firstly,ZrO₂ mill jars and balls were used to deal with the fly ashes from a grate furnace incinerator?GFI?and a fluidized bed incinerator?FBI?at Wuhan.By comparing milling process and washing process with fly ashes,it was found that the deionized water added in the milling process had little effect on the heavy metals in fly ashes,but the milling process actually had an impact on heavy metals.The leaching toxicity of heavy metals in raw fly ashes and treated fly ashes were analyzed according to the Toxicity characteristic leaching procedure,method 1311?TCLP?.And it was also concluded that mechanical milling could effectively reduce the leaching of heavy metals in fly ashes.Furthermore,the leaching concentration and the relative leaching rates?RLR?of heavy metals in fly ashes both decreased obviously.And more importantly,the leaching concentrations of all the heavy metals studied were lower than the corresponding regulatory limits after 24 h's milling.Specifically for the GFI fly ash,the decreased rate of RLRs of Cr,Cu,Zn,and Pb had exceeded over 90%.And for the FBI fly ash,the RLRs of all five heavy metals had an obvious decrease which was between 43%and 80%.In order to research the reason for the reduction of leaching,we further tested the physical and chemical properties of raw fly ashes and treated fly ashes.After milling,the size of fly ash particles reduced and the surface of particles became rough.Meanwhile,the pore volume and the average pore both increased obviously,and the specific surface area of fly ashes also became larger.In addition,the treated fly ashes had richer types of holes with more mesopores and macropores than those in the raw fly ashes,and the pore volume increased with the distribution of particle size.All of these enhanced the ability of the fly ashes to stabilize the heavy metals.The sequential extraction results of fly ashes by European Community Bureau of Reference?BCR?demonstrated that the acid-soluble fraction,the reducible fraction and oxidation fraction of heavy metals decreased.But the residual fraction which was considered not leached and more stable in natural environment increased.In general,the mechanical milling had more obvious effect on the surface of particles,the types of holes and the specific surface area of GFI fly ashes.After milling,the residual fraction of GFI fly ashes increased more than FBI fly ashes,which leaded to the better stabilization and less leaching toxicity of heavy metals in GFI fly ashes.In a word,this study made a great contribution to the research of heavy metals'stabilization in MSWI fly ashes.It was investigated that mechanical milling had a significant effect on reducing the leaching toxicity of heavy metals in MSWI fly ashes.And the milling had an impact on the speciation of heavy metals in fly ashes.In terms of explaination for the reduction,it was indicated that part of the heavy metals in fly ashes were transformed from the active fractions?the acid-soluble fraction,the reducible fraction and oxidation fraction?into the residual fraction during the milling process.Therefore,the transformation brought benefit to the stabilization of heavy metals in fly ashes.